A conventional light-receiving circuit, which converts a photo signal into an electric signal and outputs the electric signal, has such a circuit configuration as is shown in the "Light Receiver" disclosed in Published Examined Japanese Patent Application (PEJPA) No. 63-25738.
More specifically, as is shown in FIG. 1, an amplifier 102 has its input terminal 104 connected to a photodiode 100, and has its output terminal 106 connected to the positive input terminal 110 of a comparator 108. A reference potential-generating circuit 112 is provided. The output terminal 114 of this reference potential-generating circuit 112 is connected to the negative input terminal 118 of a peak value-detecting circuit 116 by way of resistor R100, and is further connected to the output terminal 106 of the amplifier 102 by way of resistors R102 and R104. The positive input terminal 120 of the peak value-detecting circuit 116 is connected to node W, which is between resistors R102 and R104. The output terminal 122 of the peak value-detecting circuit 116 is connected to node X by way of diode D100. Node X is connected to a constant current source I100. Node X is further connected to the negative input terminal 124 of the comparator 108 by way of node Y, to which a capacitor C100 is connected. The output terminal 126 of the comparator 108 is connected to the output terminal 128 of the receiver.
In the light-receiving circuit of the above circuit configuration, the threshold value is automatically set to have an optimal level, without reference to a change in photo signal E supplied to the photodiode 100.
It is proposed that the amplifier 102 to which the photodiode 100 is connected be replaced with an amplifier having differential output terminals of normal and inverted phases. Such a proposal is made, for example, in Japanese Patent Application No. 1-180717 entitled "Widely-Dynamic Light-Receiving Circuit" and Japanese Patent Application 1-311334 entitled "Light-Receiving Circuit". When the amplifier having differential output terminals is employed, it is possible to make the best use of the amplitude of photo signal E, and the amplifier can have a wide band and be widely-dynamic.
The amplifier mentioned above has differential output terminals. Thus, even if the amplifier is incorporated in a circuit wherein the threshold value is automatically set to have an optimal level, such as the circuit shown in FIG. 1, only one of its output terminals, a normal-phase one or an inverted-phase one, can be connected to a given circuit. Accordingly, the gain to be obtained is substantially 1/2 of the gain obtained in the case where both output terminals are connected to that given circuit, and the amplifier fails to sufficiently achieve its advantages.
As may be understood from the above, the circuit mentioned above is designed on condition that the amplifier 102 to be incorporated has a single-phase output terminal. The circuit is not designed for use with an amplifier having differential output terminals
As mentioned above, an amplifier having differential output terminals is not suitable for incorporation into such a circuit configuration as is disclosed in Published Examined Japanese Patent Application (PEJPA) No. 63-25738 entitled "Light Receiver".
Accordingly, an object of the present invention is to provide a light-receiving circuit which has a circuit configuration permitting an amplifier with differential output terminals to be suitably incorporated therein, which employs a wide-band, widely-dynamic amplifier, and which constantly maintains the threshold value at an optimal level without reference to a change in the photo signal.